Such a position-measuring device includes a track which has an incremental graduation extending along a measuring direction having a specified graduation period which is provided at at least one reference position with a discontinuity for generating a reference signal, as well as a scanning device, movable along the measuring direction with respect to the track for the (photoelectric) scanning of the track. In this context, the discontinuity is formed by the modification of a transverse substructure of the incremental graduation that extends perpendicular to the measuring direction, which deflects radiation beams originating from the scanning unit (by diffraction) having one component perpendicular to the measuring direction (transverse), the deflection direction at the discontinuity deviating from the deflection direction in the other regions of the incremental graduation; and the scanning unit having at least one reference pulse detector element, e.g., at least two reference pulse detector elements, which are provided having a different deflection direction for receiving radiation beams.
The perpendicular extension of the transverse substructures with respect to the measuring direction does not mean necessarily, in this context, that the transverse substructures have to extend exactly perpendicularly to the measuring direction. Rather, it is only required that the transverse substructures have at least one component in their characteristic that extends perpendicular to the measuring direction.
A position-measuring device described in PCT International Published Patent Application No. WO 03/021185 includes, for the generation of periodic incremental signals as well as at least one reference-pulse signal, a scale having a track in which a periodic incremental graduation is developed having a certain incremental graduation period, which extends along a measuring direction, and which has a discontinuity with respect to an optical property for the generation of a reference-pulse signal, as well as of a scanning unit which is movable relative to the scale in the measuring direction over a certain measuring path, and which, besides a light source, includes several detector elements for photoelectric scanning of the incremental graduation. In this context, in a first section of the measurement path, the incremental graduation has a first transverse substructure which deflects incident radiation beams into at least a first spatial direction; and in a second section of the measurement path, the incremental graduation has a second transverse substructure which deflects incident radiation beams into at least a second spatial direction that differs from the first spatial direction, so that in the transition range between the first and second section there exists a discontinuity with respect to the optical deflection action of the transverse substructures of the incremental graduation. In this context, in the various spatial directions, on the side of the scanning unit, reference-pulse detector elements are situated at which partial reference-pulse signal are present from whose processing the reference-pulse signal results.
Alternatively to the use of several (at least two) reference pulse detector elements, a reference pulse detector element being assigned to each radiation beam of a different deflection direction, a position-measuring device of the type described above can also be operated using a reference pulse detector element which is assigned to one of the radiation beams having a different deflection direction. With the aid of the illumination or non-illumination of this single reference-pulse detector element, it may then be determined whether the reference position is just being crossed.
In a conventional position-measuring device, the generation of a reference-pulse signal is derived from a reference mark that is integrated in the incremental track. For this, the graduation marks of the incremental graduation that is to be scanned have a transverse structuring (so-called transverse substructures) along their direction of extension (perpendicular to the measuring direction). The lateral boundary lines of the graduation marks are periodically modulated in the longitudinal direction, having a defined transverse graduation period. At the at least one reference position, the transverse graduation period changes and the reference mark is defined by this point of sudden discontinuity.
The transverse substructures bring about a deflection of the radiation beams impinging upon them (by diffraction) even in the transverse direction (perpendicular to the measuring direction). The degree of deflection depends, in this case, on the respective transverse graduation period, and accordingly it changes at the reference position formed by a point of sudden discontinuity of the graduation period. A certain detector element in the form of a photo element is spatially assigned respectively to the different transverse graduation periods. In the case of crossing the reference position in a relative movement of the scale with respect to the scanning unit of the position-measuring device, the deflection effect on the radiation beams changes such that by this time the respectively other detector element is acted upon. From the signal pattern (pattern of the partial reference-pulse signals at the at least two detector elements, a reference-pulse signal may then be derived by signal connection, as is explained in PCT International Published Patent Application No. WO 03/021185.
Integration of a reference mark into the incremental track may provide that greater tolerances of added construction may be allowed during assembly of the respective position-measuring devices. In this context, the arrangement described in PCT International Published Patent Application No. WO 03/021185 makes certain that the incremental signal is not interfered with by the reference mark.